The present invention relates to a torque distribution device and to a method of operation thereof, for a vehicle adapted for four wheel drive operation and incorporating a four wheel drive power transmission system, and more particularly relates to such a torque distribution device and method of operation thereof for a vehicle such as an automobile incorporating such a four wheel drive power transmission system, said torque distribution device and method providing several ratios of magnitude of actuating signal, such as the magnitude of a hydraulic fluid pressure actuating signal, versus the magnitude of torque transmission capablity provided according thereto for determining torque distribution between the front and the rear vehicle wheels.
The present invention has been described in Japanese Patent Applications Ser. Nos. 61-102805 and 61-130957 (1986), both filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies of which are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front - rear differential device is provided in order to provied a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of so called tight corneenr braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front - rear differential device is effective for achieving this result. Further, it has been practiced to provide an automatic transmission system to a vehicle which is equipped with such a four wheel drive type transmission. Such a type of structure is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 56-138020 (1981). Further, it is per se conventional to provide, to such a center differential device, a torque distribution control clutch such as a central differential control clutch, which serves for regulating the distribution of the drive torque produced by the engine of the vehicle between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination). Such a type of construction is disclosed, for example, in Japanese Patent Application Laying Open Publication Serial No. 50-14027 (1975) and in Japanese Patent Application Laying Open Publication Ser. No. 55-72420 (1980). Such a torque distribution control clutch such as a central differential control clutch is typically controlled by a hydraulically operated servo device, so that the engagement pressure of said torque distribution control clutch, i.e. the maximum amount of torque that said torque distribution control clutch can transmit, which defines the amount of torque redistribution which said torque distribution control clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), is regulated by the magnitude of an actuating hydraulic fluid pressure, Such an actuating hydraulic fluid pressure is typically provided by a control system such as a hybrid electrical/hydraulic control system which may include a microcomputer.
However, a problem that arises with such a system is that said control system typically controls the engagement pressure of said torque distribution control clutch according to various vehicle operational parameters such as for example the torque amount that is being inputted to the central differential device, the throttle opening of the engine of the vehicle (which is typically taken as a parameter representative of engine load), the vehicle road speed, the shift stage of the transmission that is currently engaged, the angle of the steering system of the vehicle, and the like. Accordingly, because of the wide variation that is possible in these vehicle operating parameters, typically it is required for the amount of torque redistribution which said torque distribution control clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), i.e. the maximum amount of torque that said torque distribution control clutch can transmit or the engagement pressure of said torque distribution control clutch, to be relatively accurately varied over a relatively large operational range. Now, conventionally the servo actuator for such a torque distribution control clutch has comprised a single piston sliding in a single cylinder bore, and has had a single pressure chamber to which the actuating hydraulic fluid pressure is supplied. Conventionally, therefore, the actuating hydraulic fluid pressure for said torque distribution control clutch has been required to be adjusted within relatively narrow limits and relatively accurate, over a relatively wide range. Since the maximum range of said actuating hydraulic fluid pressure which is provided by the control system for the servo device is inherently restricted by the value of line pressure, the accuracy of control of said actuating hydraulic fluid pressure which is required is very high, and this provides a major stumbling block with regard to implementing such a system.